Mapping Hacks

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Chapter 4, Mapping (on) the Web

#44 Plot Statistics Against Shapes

HACK

You can also use it to plot specific values using a color palette that you sup-

ply. The first technique is the more useful here.

For this hack, we’ve provided a sample shapefile, voting precincts in San

Francisco as of 2003, and an Excel file that has data that maps to it. You

should be able to use any freely available political shape you find, if you can

find or create statistics that share a key column with it.

SVG::Shapefile reads

.csv and .xls files using the Perl DBI interface, so you can hook it up directly

to an SQL database if you want.

The Code

This script assumes you have the zipped-up shapefile and the Excel file in

the same directory you run it from. Get them from http://mappinghacks.com/

data/PRECINCTS.zip and http://mappinghacks.com/data/SF_runoff.xls. Type

unzip PRECINCTS.zip to extract the contents of the shapefile, and then run

this script:

#!/usr/bin/perl

use strict;

use lib qw(/home/jo/indymapper);

use SVG::Shapefile;

my $svg = SVG::Shapefile->new(ShapeFile => 'PRECINCTS.shp',

PolygonID => 'PRECINCT',

DataFile => 'SF_runoff.xls',

KeyColumn => 'PRECINCTS',

ValueColumn => 'TURNOUT',

Colors => [[0,255,0],[255,0,0]]

);

$svg->render('map.svg');

Reading large shapefiles and converting them to SVG geometry can take

some time, so don’t worry if the script waits for quite a few seconds before

returning.

The

PolygonID